1. Field of the Invention
This invention relates to an electrostatic printing apparatus, and particularly to an apparatus for electrostatically printing images on a specific recording medium by a liquid developing process.
2. Description of the Related Art
In the conventional color image printing process by electronic photography, generally an electrostatic latent image of the original is formed on a photosensitive medium by a laser exposure system, and developed by the dry developing method using toner powder, and this developed toner image is electrostatically transferred onto a sheet. This conventional system employs the dither method wherein the desired tone grading of an image, particularly, color image to be reproduced is approximated by the adjustment of the density of binary dots. This system, however, has a problem in that it is difficult to reproduce smooth tones and that the resolution is reduced when the tone is to be reproduced with high fidelity. In addition, since toner powder of 7-10 .mu.m in diameter are used for developing the electrostatic latent image, the minimum size of the dots as reproduced is limited, and thus the resolution is also limited. As a result, it is difficult to obtain satisfactory tone grading and resolution.
There is another electrostatic latent image forming method in which the electrostatic latent image of a dot pattern showing an image to be reproduced is formed by ion flow, and the amount of ion flow for each dot of the dot pattern is controlled to achieve a desired tone grading of the reproduced image. This method or apparatus is disclosed in, for example, U.S. Pat. Nos. 3,715,762; 3,742,516; and 4,463,363. In addition, there is a liquid developing method using a solution of toner for developing latent images with high fidelity. This liquid developing method can reproduce a high-quality image since it uses smaller-diameter toner particles than the dry developing method. The combination of the electrostatic latent image forming method using ion flow and the liquid developing method can reproduce an extremely high-quality image as compared with the conventional electronic photography.
In this combined method, however, it is fundamentally difficult to satisfactorily transfer the image to ordinary paper due to the electric characteristics of toner and the powder characteristics of toner particles contained in the toner liquid. Therefore, the so-called direct recording system is widely used in which the reproduced image is formed on electrostatic recording paper which is produced by coating an insulating layer on a conductive base paper, or substrate as disclosed in, for example, "Single Path Color Electrostatic Block 2010 Series" written by Naoya Matsuda, Nippon Steel Technical Report, No. 341, Annex, Sept. 1992.
In another system, as disclosed in U.S. Pat. No. 4,686,163, a special transfer material is coated on a photosensitive medium to form a transfer sheet, and the toner image developed on the transfer sheet is transferred to ordinary paper. In this method, the electrostatic latent image can be formed by an exposure process similar to the conventional electronic photography, but it must be formed on a transfer sheet of a complicated structure having a transfer function.
Thus, the conventional electronic photography has difficulty in satisfying both high resolution and faithful tone grading. In addition, the image printing method, or the combination of the ion flow electrostatic latent image forming process and the liquid developing process encounter difficulty in satisfactory transfer. Moreover, in the method using electrostatic recording paper which is formed by coating an insulating layer on a conductive base of paper, a high-quality reproduced image can be obtained by a simple mechanical arrangement, but the image receiving paper on which the reproduced image is formed is limited.